1. Field of the Invention
This invention relates to a process for pulling a single crystal and more particularly. it is concerned with a process for pulling up a single crystal of GaAs, InP, GaP, InAs, AlAs, CdSe, CdTe, ZnTe, MnTe or HgTe or mixed crystals thereof, and an apparatus for carrying out this process.
2. Description of the Prior Art
Control of the temperature distribution in a raw material melt and the temperature fluctuation with the passage of time has hitherto been carried out by inserting a baffle plate in the raw material melt to suppress a convection in the melt.
The prior art relative to the present invention is exemplified as follows:
(1) "J, Crystal Growth" 10, 91-96 (1971), P.A.C. Whiffin and J. C. Brice PA0 (2) Japanese Patent Application OPI (Kokai) No. 3612/1972 PA0 (3) Japanese Patent Application OPI (Kokai) No. 64482/1976 PA0 (4) Japanese Patent Application OPI (Kokai) No. 188500/1982 PA0 (5) Japanese Patent Application OPI (Kokai) No. 15097/1983 PA0 (6) "J. Crystal Growth", 65, 237-242 (1983), D. Mateika, R. Laurien and M. Liehr
The above described References 1 and 2 show that the temperature fluctuation is suppressed and the radial temperature distribution is flattened in a raw material melt to grow a strain-free homogeneous crystal by providing a baffle plate substantially vertically to the pulling direction of the crystal in a range of 1/4 to 3/4 of the depth of the raw material melt from the bottom of a crucible (FIG. 7).
Reference 3 describes that a baffle plate having an opening at the central part is floated on a raw material melt and a single crystal is pulled through the opening to control the shape of the single crystal, the baffle plate being made of boron nitride (BN), quartz (SiO.sub.2), graphite (C), silicon nitride (SiN) or silicon carbide (SiC).
Reference 4 discloses the use of a sintered body of Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 type or lanthanide element oxide-Al.sub.2 O.sub.3 type, excellent in corrosion resistance at high temperatures and mechanical strengths, as a material of a baffle plate.
Reference 5 describes an apparatus in which a baffle plate is provided on a raw material melt and supported at a suitable position by a guide pin.
Reference 6 discloses that a single crystal of Nd.sub.3 Ga.sub.3 O.sub.12 is grown by devising the position of an opening provided in a baffle plate and thereby controlling the temperature distribution in the radius direction to a desirable distribution, as shown in FIG. 8.
In the case of flattening the radial temperature distribution of a melt as shown in References 1 and 2, the so-called seeding and growth of the shoulder or cone part are so unstable that the diameter of a pulled crystal often fluctuates.
In the case of References 3, 4 and 5, these inventions are made to suppress the diameter fluctuation for the purpose of making a crystal with a constant trunk diameter and pulling of the straight trunk part can stably be carried out but prevention of the unstable operation from seeding to growth of a shoulder or cone part, as proposed by References 1 and 2, is impossible. Thus, these inventions have a disadvantage that the crystal growth of a cone part is so unstable that an abnormal growth takes place, resulting in frequent formation of twin crysatals or polycrystalline substances, and a high quality single crystal is hardly obtained.
In the case of crystals which tend to give a convex solid-liquid interface during growth, as described in Reference 6, on the other hand, pulling of the crystal is made possible by using a baffle plate and holding the temperature lower at the center of a crucible than at the circumference thereof, but as the temperature of the melt is gradually lowered because of increase of the diameter of the pulled crystal, there is a remarkable tendency for the solid-liquid interface to become concave so that the resulting crystal is not homogeneous.
In the prior art as described above, a baffle plate has always been used in such a manner that the diameter of the surface of a raw material melt in the baffle plate is held constant, and change thereof has not been carried out with the increase of the diameter of a pulled crystal. Therefore, the effect of the baffle plate cannot sufficiently be exhibited throughout the process of pulling.
In order to solve the above described problem, it is required to control best the temperature distribution of a melt in the radius direction with the increase of the diameter of a crystal during pulling.